1. Field of the Invention
The present invention relates to vehicular radar systems, and more particularly to an apparatus, method, and article of manufacture for a vehicle collision avoidance system which determines whether an obstacle lies in the path of a host vehicle.
2. Description of the Related Art
There is a continuing need to increase the density of vehicles traveling the world's roadways and simultaneously to improve the safety of highway vehicle operations by preventing highway vehicles from colliding with stationary and moving objects (such as roadside obstacles and other vehicles). One means for accomplishing these seemingly contradictory goals is to monitor the relative speed, direction of travel, and distance between a vehicle and any obstacle in its path, and to use such information to provide warnings of potential danger to the vehicle's driver. It is becoming increasingly more common for automotive engineers to consider the use of radar systems as a means to monitor such environmental conditions.
Vehicle borne radar systems which transmit and receive at three different frequencies on a time division basis, with two of the frequencies being used to determine range, and the third being combined with one of the first two to determine closing speed and likelihood of collision, are presently known. One such system is disclosed in U.S. Pat. No. 3,952,303 to Watanabe et al., which teaches an analog radar system processing front end.
Another example of an automotive radar system is described in U.S. Pat. No. 5,402,129, entitled Multi-Frequency Automotive Radar System, and assigned to the assignee of the present invention. In that system, a transmit signal and the reflected received signal are coupled to an RF mixer. The relevant output from the RF mixer is a signal that has a frequency equal to the difference between the transmit and receive frequencies. The frequency of the reflected received signal may be shifted from the frequency of the transmit signal upon its return due to the "Doppler" effect. Doppler effect occurs whenever a transmitted signal reflects off an obstacle that has a motion relative to a transceiver. The resulting frequency shift is referred to as a "Doppler shift".
A further example of an automotive radar system, which uses principally a digital approach, is described in U.S. Pat. No. 5,302,596, entitled Multi-Frequency, Multi-Target Vehicular Radar System Using Digital Signal Processing, and assigned to the assignee of the present invention. In that system, which includes a transmit section for generating two-channel transmit frequency, an antenna both transmits the transmit signal and receives a reflected receive signal. A diode mixer generates a difference signal having a frequency equal to the transmit frequency minus the received frequency. A signal switch in a front end electronics section time multiplexes and samples the channel one and channel two signals, following which the samples are coupled to a two-channel analog-to-digital converter. A digital electronic section receives the digital information and performs a Fast Fourier Transform on each channel of digital data to determine relative speed and range of an obstacle based upon the frequency and the difference in phase of the two channels. The digital electronic section also receives information regarding the status of vehicle operation and/or controls to determine the degree of danger presented by an identified obstacle.
In vehicular collision avoidance radar systems, it is necessary to know whether or not an obstacle is in the host vehicle's path of travel. This is typically done by determining whether the obstacle is off-boresight, and if so, the direction and amount of the angular error (angular deviation from boresight). It is also desirable or necessary to know the distance or range of the obstacle. To provide for continuous tracking, a number of systems have been proposed including those which transmit a signal and then combine a multiplicity of diversely received replicas of the signal. Examples of such systems are provided by U.S. Pat. No. 4,060,809 of Baghdady, U.S. Pat. No. 4,975,710 of Baghdady, U.S. Pat. No. 5,084,709 of Baghdady, and U.S. Pat. No. 5,128,969 of Baghdady. However, such systems have proven to have shortcomings which make them undesirable or impractical when employed for use in vehicular collision avoidance radar systems. Such systems are limited in their ability to provide data when the host vehicle with the radar system is on a curved road, since the mere knowledge of the angular error is insufficient to allow the collision avoidance radar system to predict the path of the host vehicle with respect to the location of the obstacle. Accordingly, there is a need for a system that can so predict the path of the host vehicle with respect to obstacles identified by the system.